In recent years, with the significant progress in information network technology and expanding services that implement information networks, there has been a demand for increased data volumes and faster processing speeds for electronic devices. Smaller signal wavelengths are effective for transferring digital signals in greater volumes and at higher speeds, and advances are being made toward achieving higher signal frequencies. Because electrical signals in the high-frequency range tend to decay in wiring circuits, there is a need for electronic circuit boards with high transmission characteristics.
Two approaches are effective for obtaining electronic circuit boards with high transmission characteristics, namely (i) reducing the dielectric loss tangent of the dielectric material (such as the insulating resin reinforced in the base material) and (ii) lowering the skin resistance of the conductor (such as metal wiring).
As a method of (i) reducing the dielectric loss tangent of the dielectric material, there is known a method using a low-dielectric resin such as polyphenylene ether (hereunder, PPE) as the insulating resin. PPE has low permittivity, a low dielectric loss tangent and an excellent high-frequency characteristic (i.e. dielectric characteristic), as well as high heat resistance, and is therefore suitable as an insulating material for electronic circuit boards of electronic devices that utilize high frequency bands.
In PTL 1 there is disclosed a technique of adding a curable monomer or polymer to PPE to form a curable resin composition. In PTL 2, there is disclosed a method of chemically modifying PPE and combining the PPE with triallyl isocyanurate and/or triallyl cyanurate as a curable monomer to form a curable resin composition. PTLs 3 and 4 disclose PPE resin compositions employing low-molecular-weight PPE. Also, PTLs 5 and 6 describe methods using opaque dispersions in which particles of a resin composition comprising a crosslinkable resin such as PPE or styrene-butadiene copolymer and a crosslinking aid such as triallyl isocyanurate (TRIC) are dispersed in a non-chlorine-based organic solvent at ordinary temperature. PTL 7 describes a resin composition comprising low molecular weight PPE and an epoxy resin. PTL 8 describes a method employing a varnish in which PPE particles are dispersed in a solvent consisting of at least 90% water.
PTL 9 describes a low-dielectric resin comprising a PPE-modified butadiene polymer, an inorganic filler and a saturated thermoplastic elastomer. PTL 10 describes a high-frequency multilayer wiring board employing PPE as a low-dielectric resin. PTL 11 describes a laminating material employing a thermosetting resin comprising a resin with a polyphenylene oxide backbone and TRIC, as a low-dielectric resin.